1. Field of the Invention
The present invention relates to a valve mechanism for use in a hydraulic damper, and more particularly to a one-way damping valve mechanism for a hydraulic damper for use in an automobile, a motorcycle, and the like.
2. Description of the Relevant Art
FIG. 23 of the accompanying drawings illustrates a known valve mechanism of the Sachs type for use as a one-way damping valve mechanism for a hydraulic damper for use in an automobile, a motorcycle, and the like. The valve mechanism is disposed between a piston rod 101 inserted in a cylinder 100 and a piston 103 fixed to a shank 102 of the piston rod 101. The piston 103 has a plurality of axial oil holes 103a defined therein. The interior space of the cylinder 100 is divided by the piston 103 into an upper hydraulic chamber S1 and a lower hydraulic chamber S2. A valve guide 104 having a plurality of axial oil holes 107a is interposed between a valve collar fitted over the shank 102 and the piston 103. In the valve guide 104, there is slidably fitted a valve seat 107 having a plurality of axial oil holes 107a. A valve 106 comprising a plurality of annular disc valve members is fitted in the valve guide 104 beneath the valve seat 107. The valve members of the valve 106 have central holes with their radius smaller than the radius of the central hub of the valve seat 107. A spring 108 having a relatively small spring constant is disposed between the upper surface of the valve seat 107 and the valve guide 104 for normally urging the valve seat 107 to move downwardly in FIG. 23. The lowermost valve member 105 of the valve 106 has a plurality of slits 105a defined in its outer peripheral edge.
The valve mechanism of the aforesaid construction generates a damping force during an expansion stroke when the piston 103 moves upwardly in FIG. 23 and produces substantially no damping force during a compression stroke when the piston 103 moves downwardly in FIG. 23. More specifically, when the piston 103 moves upwardly at a low speed that is 0.1 to 0.3 m/s in the expansion stroke, the valve 106 is not substantially flexed, and working oil in the upper hydraulic chamber S1 flows into the lower hydraulic chamber S2 through the slits of the valve guide 104, the slits 105a of the lowermost disc valve member 105, and the oil holes 103a. At this time, a prescribed damping force is generatd dependent on the resistance imposed on the oil flow by the slits 105a. In this condition, the upper surface of the inner periphery of the valve 106 remains in engagement with the lower surface of the inner periphery of the valve seat 107, with no oil passage defined therebetween. When the piston 103 moves upwardly at a medium or high speed this is, greater than 0.3 m/s in the expansion stroke, the inner periphery of the valve 106 is flexed downwardly under a hydraulic pressure buildup in the hydraulic chamber S1, defining an oil passage between the valve seat 107 and the valve 106. Working oil in the upper hydraulic chamber S1 flows into the lower hydraulic chamber S2 through the oil passage between the valve seat 107 and the valve 106 as well as the slits 105a. At this time, a relatively large damping force is generated by the valve members of the valve 106. The damping force generated when the piston 103 moves at a low speed can appropriately be selected by adjusting or varying the cross-sectional area of the slits 105a. When the piston 103 moves downwardly in the compression stroke, the valve 106 and the valve seat 107 are displaced upwardly against the bias of the spring 108, so that the working oil in the lower hydraulic chamber S2 is caused to flow through the oil holes 103a and the slits of the valve guide 104 into the upper hydraulic chamber S1. At this time, virtually no resistance to the oil flow is present, and virtually no damping force is produced.
Heretofore, the sliding surfaces of the hydraulic damper such as the outer circumferential surface of the piston 103 and the inner circumferential surface of the cylinder 100 are subject to a relatively large degree of frictional resistance which acts as a damping force when the piston 103 moves upwardly at the low speed during the expansion stroke. This damping force is effective to give suitable riding comfort during low-speed travel of the piston 103 in the expansion stroke. However, various recent improvements have resulted in reduced coefficients of friction of such sliding surfaces. Therefore, the damping force produced when the piston moves at the low speed in the expansion storke tends to be so reduced that any suitable riding comfort can be obtained by the damper. This problem could be solved by reducing the cross-sectional area of the slits 105a of the disc valve member 105 of the valve 106. However, only such a solution would fail to compensate for a shortage of damping force when the piston would move at an extremely low speed this is, less than 0.1 m/s. Moreover, no smooth damping transient would be achieved since the damping force would abruptly be increased when the piston speed would change from the extremely-low-speed/low-speed range to the medium-speed/high-speed range.
Japanese Laid-Open Patent Publication No. 56-20846 published Feb. 26, 1981 discloses a valve mechanism having a disc-shaped subvalve disposed below the valve 106 of FIG. 23 for generating a required damping force when the piston moves at the extremely low speed or the low speed. The disclosed valve mechanism controls the damping force in multiple stages when the piston moves at low speeds. The subvalve has disc valve members with their inner peripheries limited against upward movement by the outer peripheral edge of a stopper fixed to the valve collar. During the compression stroke of the piston, the outer periphery of the disc valve members of the subvalve are flexed to define an oil passage between the subvalve and the piston. Consequently, this valve mechanism produces a damping force also during the compression stroke of the piston. The valve mechanism disclosed in the above publication is not suitable for use as a one-way damping valve mechanism which is required to produce a damping force only when the piston moves in one direction.
The present invention has been made in an effort to eliminate the problems experienced by the conventional one-way damping valve mechanism as shown in FIG. 23 of the accompanying drawings.